Trigonally distorted systems coupling

Branched P3 donor ligands such as MeC(dppm)3, MeP(dppe)2, PhP(dppe)2 and PhP(dppp)2 form distorted square-pyramidal low-spin monomeric [Co(X)2(L)] (X = Cl-, Br-) or [CoX(L)]+ (X = N03-, AcO-, acac-) complexes, e.g. (152), or dimeric trigonal-bipyramidal systems [CoX(L)]2+ (153) in which the low magnetic moment is believed to result from exchange coupling via the X bridges.509 Table 39 gives some representative preparations. Monomeric [Co(X)2(L)] is sensitive towards 02 and is inclined to dissociate to tetrahedral species in solution, whereas the dimeric systems are more stable. 

To achieve a bonding interaction between the 7C-systems of two ligands, two modes of coupling were considered not to require a too important distortion. Due to the trigonal bipyramid structure with the five-membered ring in equatorial-apical orientation, the two interactions A = ei-ai and B = ei-a2 were discarded. On the other hand, the least-motion most favoured interactions were considered to be C = ai-e2, D = a2-C2 and, to a lesser degree, E = ei-e2- ... 

It is, of course, highly artificial to consider each of the above-mentioned coupling cases in isolation. For example, in considering Tig - Tiu interactions rigorously it would be necessary to take into account simultaneously the likelihood that both Tig(A2 + Ea) and Tiu(A2 + E ) are both JT and PJT distorted. The same would hold true for vibronically induced Tig - T2g interactions. Such considerations would lead to exceedingly tedious and complicated formal manipulations and horrendous computational requirements, if one s objective was to explicate quantitatively the vibronic details of the Aig - Tig + T2g CD spectra of trigonal Co(III) systems. At present it would appear necessary to be content with the qualitative conclusions elicited from the less-than-complete treatments given in this section. 

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